Mooring means

ABSTRACT

A mooring system comprises a sheave (12) adapted to be mounted to a base (11) which is located on the sea bed. In one embodiment, a cable (13) received in the sheave (12) is connected at one end to a first buoyant member (15) and is connected at its other end to a second buoyant member (23). The buoyancy of the second buoyant member (23) is less than that of the first buoyant member (15). Mooring energy caused by movement of a vessel moored to the second buoyant member is absorbed by the combination of a resistance to movement of the second buoyant member (23) as it is pulled away from a vertical position adjacent the cable (13) and the buoyancy of the first buoyant member (15) as it is pulled downwards by the cable (13) towards the sheave (12).

This invention relates to moorings for water borne vessels.

Conventional moorings comprise a base which is fixed to the sea bed anda length of chain or the like is fixed at one end to the mooring whilethe free end of the chain is fixed to a mooring line supported from thesurface of the water by a buoy for attachment to the mooring line of avessel when required. On attachment of a vessel's mooring line to thecable, the base and chain serve to prevent movement of the vessel awayfrom the mooring. The function of the chain is to provide an theinertial load created by the movement of the vessel away from themooring as a result of water conditions and provide a reaction to theforces applied by the vessel in addition to the restraint by the base.As the load applied by the vessel increases so the chain will be liftedfrom the sea bed. When maximum load has been applied by the vessel, thechain is lifted free of the sea bed and the load of the chain is fullyapplied to the base. The arrangement as described above is also relevantto anchors which are conventionally provided with a length of chainwhich is attached to the end of the anchor rope whereby the chain servesto absorb at least some of the load applied by the vessel when in themoored condition and to absorb some of the inertial loads.

A difficulty with arrangements as described above relates to the amountof space that is required to be provided between moorings in order toprovide for the free movement of a vessel under extreme conditions. Suchmoorings also create a difficulty in that as the vessel swings about themooring, as a result of a change in wind, tidal or wave conditions, sothe chain is dragged over the sea bed around the mooring. This erosionof the sea bed around the mooring base serves to destroy any sea grass,coral and other marine life that may be in the region over which thechain is dragged. This erosion also results in the continual disturbanceof the sea bed, the effect of which disturbance is to introduce into thewater, a suspension of sediments, nutrients and any pollutants that maybe retained in the sea bed.

It is an object of this invention to provide a mooring system which isable to absorb the inertial loadings that may be applied to the mooring;which is able to accommodate the loadings that may be applied to themooring as a result of movement of a moored vessel under extremeconditions; and which reduces the degree of disturbance of the sea bedin the region surrounding the mooring.

Throughout this specification the term "sea bed" shall be taken toinclude the bottom of any body of water in any aquatic environment.

SUMMARY OF THE INVENTION

According to the present invention there is provided a mooring systemfor mooring a floating vessel, the mooring system comprising:

a sheave assembly adapted to be anchored to a base located on theseabed;

a cable received in the sheave assembly;

a first buoyant member connected to a first length of the cableextending from one side of the sheave assembly and adapted to float onthe surface of the water when the cable is in an unloaded condition;

a second buoyant member connected to a second length of the cableextending from the other side of the sheave assembly and having abuoyancy less than that of the first buoyant member, and wherein whenthe cable is in an unloaded condition the second buoyant member isadapted to lie substantially submerged below the surface of the wateradjacent the first length of cable to maintain the first and secondlengths of cable in a substantially vertical orientation and tautcondition;

whereby, in use, when a mooring line of the vessel is connected to saidsecond buoyant member, mooring energy caused by movement of the vesseland transferred to the mooring system can be absorbed by the combinationof a resistance to movement of the second buoyant member as it is pulledaway from a vertical position adjacent the first length of cable and thebuoyancy of the first buoyant member as it is pulled downwards by thecable towards the sheave assembly.

According to a preferred feature the first buoyant member comprises aplurality of buoyant elements which are mounted sequentially on thecable. In addition it is preferred that the buoyancy of at least some ofthe buoyant elements increase with their spacing from the sheave.

In a preferred form the buoyant elements are supported on a rod elementfixed to the other end of the cable. The length of the rod element maybe greater than the combined length of the buoyant elements whereby thebuoyant elements are slidable along the rod element and where the firstbuoyant element is limited in its extent of slidable movement along therod element.

According to a further preferred feature, the second buoyant member maycomprise a plurality of buoyant members supported along the cable.According to a further preferred feature, the buoyant members are spacedfrom each other and a weight is mounted to the cable intermediate eachbuoyant member.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood in the light of thefollowing detailed description of preferred embodiments of the mooringsystem, given by way of example only. The description is made withreference to the accompanying drawings of which:

FIG. 1 is a schematic view in an unattached mode of the mooring meansaccording to a first embodiment;

FIG. 2 is elevation of the first buoy when attached to moored vessel;

FIG. 3 is a schematic view of a second embodiment;

FIG. 4 is a part schematic view of a third embodiment showing theretaining line; and

FIG. 5 is a schematic illustration of a fourth embodiment of the mooringsystem having an alternative form of the second buoy.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The mooring system according to a first embodiment is intended to bemounted to a base 11 which is located in the sea bed. The base maycomprise any suitable means having sufficient mass and may comprise aconventional anchor with a length of anchor chain attached to it wherethe mooring system is mounted to the end of the chain remote from theanchor.

The mooring system according to this embodiment comprises a sheave 12which is anchored to the base 11 and cable 13 which is received throughthe sheave. The sheave 12 is mounted to the base 11 through a pivot orswivel. One end of the cable is formed with an eye 14 to facilitateattachment of the mooring line of a vessel thereto. The other end of thecable 13 has a first buoy 15 fixed to it.

The first buoy 15 comprises an elongate rod 16 having an eye andassociated swivel 17 mounted to its lower end which is fixed to theother end of the cable 13. The other end of the rod 16 is provided witha circular stop plate 18. The rod element 16 slidably supports a pair ofbuoyant elements 19 and 20 which are mounted in a series along the rod16. The length of the rod 16 is greater than the combined length of thebuoyant elements 19 and 20 and the buoyant elements are capable ofslidable movement along the rod 16. A second stop plate 22 is mounted tothe rod intermediate of the first and second buoyant elements 19 and 20respectively and serves to limit the degree of slidable movement of thefirst buoyant element 19 along the rod 16. The first buoyant element 19has a cylindrical configuration.

A second buoy 23 is mounted to the cable intermediate of the sheave 12and the eye 14. The second buoy 23 is formed to have a rod-likeconfiguration and incorporates a rod which extends through the buoy andis adapted at each end to be fixed into the cable 13. The second buoy 23has a buoyancy less that the first buoy 15. As a result under no loadconditions the second buoy will be submerged and lie closely adjacentthe portion of the cable 13 between the sheave 12 and the first buoy 15.

In use when a mooring bridle of a vessel has been affixed to the eye 14on the cable 13, the load which is initially applied to the cable 13will be dissipated initially by the second buoy 23 and the first buoyantelement 19. This dissipation will be effected by the second buoy beingpulled away from the vertical position adjacent the cable 13 and thusbeing pulled under the water. In addition, the first buoyant elementwill also be pulled under the water to a limited extent. As the loadwhich is applied by the mooring line increases the portion of the cablebetween the sheave 12 and the eye 14 will tend to straighten. As theload increases further the first buoy will be pulled downwardly towardsthe sheave 12 by the cable. The resultant submergence of the firstbuoyant element 19 will increase the counteracting force applied by thefirst buoy 15. Once the first buoyant element 19 has been fullysubmerged the degree of counteracting force applied by that buoyantelement will remain constant and a further increase in the counteractingforces will applied by the first buoy will be as a result of the secondbuoyant element 20. The further counteractive force will only come intoeffect when the rod has been pulled downwardly to an extent such thatthe first stop plate 18 engages the upper surface of the second buoyantelement 20 which will cause the second buoyant element to be pulled intothe water.

The degree of buoyancy provided by the first buoy is sufficient to beable to accept an anticipated load desired of a particular mooring underthe very worst conditions and as a result of such extreme conditionswill the first buoy will become fully submerged. In the event of such aninstance a suitable stop member may be provided along the cable 13adjacent the junction with the rod element 16 to engage sheave andprevent any damage to the swivel and eye assembly which provides theattachment for the cable to the first buoy.

Under low tide conditions, the first buoy will float on the surface ofthe water and the second buoy 23 will be submerged and will be adjacentthe cable 13 between the sheave 12 and the first buoy 15. Under hightide condition the first buoy 15 will cause the second buoy 23 to bepulled closer to the sheave 12.

The function of the embodiment as described above is to provide the samecatenary through a mooring line as is conventionally applied by aconventional mooring line and chain, however, in so doing there is nocontact between the mooring system and the sea bed. This results in noerosion of the sea bed and destruction of marine growth in the vicinityof the mooring as a result there is no introduction of sediments,nutrients and pollutants from the sea bed into the water by the actionof mooring.

The number of buoyant elements which form the first buoy 15 may bevaried in order to increase or decrease the degree of buoyancy.Furthermore, the buoyant elements may take any desired configurationincluding one which will provide a non-linear increase in thecounteracting force being applied by the first buoy 15 against the loadwhich is to be applied to the cable 13 by a moored vessel attached tothe mooring.

In an alternative arrangement the second buoy 23 may comprise aplurality of buoyant members mounted in series along the cable 13 andwhich are able to float on the surface. As a result, when a load isapplied by the mooring line to the cable 13, each of these buoyantmembers will be consequently submerged to provide a graduated increasein the reaction force applied to the load being applied by the vessel. Avariation of this form of second buoy is shown at FIG. 5 where thebuoyant members 25 are located at spaced intervals along the cable 13and where a weight 26 is mounted to the cable midway between each of thebuoyant members 25. Under no load conditions, the biasing force appliedby the weights 26 will cause the buoyant members to be pulled to closeabutting relationship with each other. Any load which is applied to thecable 13 by a moored vessel will initially need to counteract the effortapplied by the weights in order to cause separation of the secondbuoyant members 25 prior the second buoyant members being submerged.

According to a second embodiment which is a variation of the firstembodiment and which is shown at FIG. 3 the portion of the cable 13between the second buoy 23 and the one end may be replaced by a rigid orsemi rigid rod 30 having the eye 14 at its outer and a float 31 wherebythe eye 14 is held above the surface of the water to facilitate theretrieval of the mooring.

According to a third embodiment shown at FIG. 4, the mooring of thefirst and second embodiments are provided with a retaining line 32 fixedbetween an eye 33 provided on the sheave or if desired on tile base 11and the lower end of the second buoy 23. The retaining line 32 has alength such that under the very worst conditions the retaining line willprevent the first buoy 15 from being fully submerged. The retaining line32 is provided with a small float 34 of intermediate length which holdthe retaining line clear of the sea bed and sheave when it is slack. Theretaining line 32 also acts as a safety wire should the mooring cable 13fail due to fatigue or wear.

In each of the embodiments, the cable 13 may be readily cleaned ofmarine growth by disconnecting the cable from one or other of the buoysand drawing the cable through the sheave assembly.

As can be seen most clearly in FIG. 4 the preferred embodiment of thesheave assembly comprises a grooved pulley or sheave 12 rotatablymounted between first and second plates 34. First and second arms 38, 40are connected to the sheave assembly and are each provided with a block42 at the free end thereof. The mooring cable 13 is threaded through ahole in the block 42 on the first arm 38, it then passes around thesheave and back up through a hole in the block 42 on the second arm 40.

Under adverse weather conditions additional buoyant elements can beapplied to the first buoy in order to increase the capacity of themooring.

In comparing the mooring system of the above described embodiments witha conventional mooring the following advantages are provided:

1. A counteracting tension is provided by the second buoy against thefirst buoy which serves to retain all of the pendant assembly of themooring line above the sea bed floor. As a result no moving partsradiate around the mooring. This serves to minimise the damage to seagrass and disturbance of the sediment in the sea bed.

2. The first buoy serves to provide a continual reaction force againstany load which is being imposed upon it. Only when the total buoyancy ofthe first buoy has been overcome and the line and the cable 13 has beenfully drawn under will the moored vessel use its maximum swing.

3. As the inertial force and load created by a moored vessel diminishes,the reaction force provided by the second buoy will cause the secondbuoy to move towards a position adjacent the first buoy and thus recoverthe swinging room of the vessel to result in a centering effect.

4. The mooring system of the described embodiments is less massive thanthat of conventional moorings which use heavy chain.

5. The system also requires less joining and wear points thanconventional assemblies.

6. The elongated shape and buoyancy of the second buoy act to prevententanglement of the mooring cable.

It should be appreciated that the scope of the present invention shouldnot be limited to the particular scope of the embodiments describedabove. In addition, it will be appreciated that any reference to sea bedor marine conditions will be taken to apply to any aquatic environment.

I claim:
 1. A mooring system for mooring a floating vessel, the mooringsystem comprising:a sheave assembly adapted to be anchored to a baselocated on a seabed; a cable freely passing through the sheave assembly;a first buoyant member connected to a first length of the cableextending from one side of the sheave assembly and adapted to float on asurface of water above said sheave assembly when the cable is in anunloaded condition; a second buoyant member connected to a second lengthof the cable extending from the other side of the sheave assembly andhaving a buoyancy less than that of the first buoyant member, andwherein when the cable is in an unloaded condition the second buoyantmember will lie substantially submerged below the surface of the wateradjacent the first length of cable to maintain the first and secondlengths of cable in a substantially vertical orientation and tautcondition; and means for connecting a mooring line of the vessel to saidsecond buoyant member, so that mooring energy caused by movement of thevessel and transferred to the mooring system is absorbed by thecombination of a resistance to movement of the second buoyant member asit is pulled away from a vertical position adjacent the first length ofcable and the buoyancy of the first buoyant member as it is pulleddownwards by the cable and towards the sheave assembly.
 2. A mooringsystem as claimed in claim 1, wherein the second buoyant member iselongate and is fixed at one end to the second length of cable.
 3. Amooring system as claimed in claim 1, wherein the first buoyant memberhas a configuration whereby at least a portion increases incross-sectional area between a lower and upper end.
 4. A mooring systemas claimed in claim 1, wherein the first buoyant member comprises aplurality of first buoyant elements mounted sequentially one above theother.
 5. A mooring system as claimed at claim 4, wherein of any pair ofadjacent first elements, the upper most first buoyant element has agreater volume that the lower adjacent first buoyant element.
 6. Amooring system as claimed in claim 5, wherein the first buoyant elementsare supported on a rod element connected to said first length of thecable.
 7. A mooring system as claimed in claim 6, wherein the length ofthe rod element is greater than the combined length of the first buoyantelements and wherein the first buoyant elements are slidable along therod element.
 8. A mooring system as claimed in claim 7, wherein thedegree of slidable movement of the lowermost first buoyant element isless than the degree of slidable movement of the other first buoyantelements.
 9. A mooring system as claimed in claim 1, wherein said sheaveassembly includes a sheave pulley rotatably mounted between first andsecond plates and said sheave assembly is pivotally mounted to the basethrough a swivel.
 10. A mooring system as claimed in claim 1, wherein arod-like element is connected to said second buoyant member and issupported by a float adapted to maintain one end of the rod-like elementabove the surface of the water for connection to a mooring line of thevessel.
 11. A mooring system as claimed in claim 1, wherein a retainingline is fixed between the base and the second buoyant member, saidretaining line having a length sufficient to prevent the first buoyantmember from being pulled into engagement with the sheave.
 12. A mooringmeans as claimed at claim 11, wherein the retaining line has a lengthsufficient to prevent the first buoyant member from being fullysubmerged.
 13. A mooring system as claimed in claim 1, wherein the meansfor connecting a mooring line of the vessel is located between thesecond buoyant member and the vessel.